Polymer electrolyte fuel cells are batteries that generate electricity and heat simultaneously by allowing a fuel gas such as hydrogen and an oxidant gas such as air to electrochemically react with each other on gas diffusion electrodes serving as an anode and a cathode. FIG. 34 shows a typical structure of such polymer electrolyte fuel cell. As shown in FIG. 34, a fuel cell 100 comprises at least one unit cell (cell) consisting mainly of a membrane electrode assembly (MEA) 105 and a pair of separator plates for sandwiching the membrane electrode assembly 105, namely, an anode-side separator 106a and a cathode-side separator 106b. 
The membrane electrode assembly 105 has a structure in which a polymer electrolyte membrane 101 that selectively transports cations (hydrogen ions) is disposed between an anode 104a and a cathode 104b. The anode 104a comprises at least a catalyst layer 102a disposed in close contact with the polymer electrolyte membrane 101 and a gas diffusion layer 103a disposed between the catalyst layer 102a and the anode-side separator 106a. The cathode 104b comprises a catalyst layer 102b disposed in close contact with the polymer electrolyte membrane 101 and a gas diffusion layer 103b disposed between the catalyst layer 102b and the cathode-side separator 106b. 
The catalyst layers 102a and 102b are layers composed mainly of a conductive carbon powder carrying an electrode catalyst (e.g., platinum metal). The gas diffusion layers 103a and 103b are layers having gas permeability and electrical conductivity. The gas diffusion layers 103a and 103b are obtained by, for example, forming a conductive water repellent layer composed of a conductive carbon powder and fluorocarbon resin on a conductive porous substrate made of carbon.
As shown in FIG. 34, from the viewpoint of disposing gaskets 109a and 109b for preventing gas leakage, the MEA 105 has a structure in which the main surface of the polymer electrolyte membrane 101 is larger than those of the anode 104a and the cathode 104b, and the entire periphery of the polymer electrolyte membrane 101 extends outwardly beyond the peripheries of the anode 104a and the cathode 104b. In this specification, the periphery of the polymer electrolyte membrane 101 that extends outwardly beyond the peripheries of the anode 104a and the cathode 104b may sometimes be called “protruding portion” (the letter P in FIG. 34.
The anode-side separator 106a and the cathode-side separator 106b are conductive and serve to mechanically fix the MEA 104 and to electrically connect adjacent MEAs 104 in series to each other in a stack comprising a plurality of MEAs 104 stacked. The anode-side separator 106a has a gas channel 107a on one surface thereof (i.e., on a main surface of the anode-side separator 106a to be in contact with the anode 104a). Likewise, the cathode-side separator 106b has a gas channel 107b on one surface thereof (i.e., on a main surface of the cathode-side separator 106b to be in contact with the cathode 104b). The gas channels 107a and 107b serve to remove a gas containing an electrode reaction product and unreacted reaction gas to the outside of the MEA 104.
Further, the anode-side separator 106a and the cathode-side separator 106b have cooling fluid channels 108a and 108b formed on the other surfaces thereof, respectively. The cooling fluid channels 108a and 108b serve to introduce a cooling fluid (e.g., cooling water) for adjusting the cell temperature to a certain level. By allowing the cooling fluid to circulate between the fuel cell and a heat exchanger disposed outside the fuel cell, a thermal energy generated by the reaction can be utilized in the form of warm water or the like.
For the sake of simplification of production process, the gas channels 107a and 107b are usually formed by providing grooves on the main surfaces of the anode-side separator 106a and the cathode-side separator 106b to be in contact with the anode 104a and the cathode 104b, respectively. The cooling fluid channels 108a and 108b are usually formed by providing grooves on the outer main surfaces of the anode-side separator 106a and the cathode-side separator 106b, respectively.
In a so-called stack type fuel cell (fuel cell stack) comprising a plurality of MEAs 105 stacked and connected in series with the anode-side separators 106a and the cathode-side separators 106b interposed between each adjacent MEAs 105, there is formed a manifold for branching a reaction gas to be supplied to the fuel cell and supplying the reaction gas to each of the MEAs 105 (a manifold formed by connecting manifold apertures for supplying reaction gas and manifold apertures for exhausting reaction gas formed in the anode-side separators 106a and the cathode-side separators 106b that are sequentially laminated (not shown)).
There is also formed another manifold for branching a cooling fluid to be supplied to the fuel cell and supplying the cooling fluid to each of the MEAs 105 (a manifold formed by connecting manifold apertures for supplying cooling fluid and manifold apertures for exhausting cooling fluid formed in the anode-side separators 106a and the cathode-side separators 106b that are sequentially laminated (not shown)). Such manifolds formed inside a fuel cell are called internal manifolds. Internal manifold type fuel cells as described above are the most common type.
In the fuel cell 100, in order to prevent leakage of reaction gas (e.g., leakage of fuel gas to the cathode 104b, leakage of oxidant gas to the anode 104a, leakage of reaction gas to the outside of the MEA 105), a pair of opposing gaskets having gas sealing function, namely, an anode-side gasket 109a and a cathode-side gasket 109b, are disposed on the periphery (i.e., the periphery of the polymer electrolyte membrane 101 that is positioned outside the anode 104a and the cathode 104b) of the MEA between the opposing anode-side separator 106a and cathode-side separator 106b. 
As the anode-side gasket 109a and the cathode-side gasket 109b, for example, O-rings, rubber sheets, composite sheets composed of an elastic resin and a rigid resin, etc. are used. From the viewpoint of ease of handling of MEA 105, gaskets comprising a composite material having a certain rigidity are often combined with the MEA 105 for use.
By disposing the anode-side gasket 109a and the cathode-side gasket 109b such that they sandwich the entire protruding portion of the polymer electrolyte membrane 101, a single enclosed space that surrounds the anode 104a is formed by the anode-side separator 106a, the polymer electrolyte membrane 101 and the anode-side gasket 109a. Likewise, another enclosed space that surrounds the cathode 104b is formed by the cathode-side separator 106b, the polymer electrolyte membrane 101 and the cathode-side gasket 109b. These enclosed spaces function to prevent the reaction gases supplied to the anode 104a and the cathode 104b from leaking.
When disposing the anode-side gasket 109a and the cathode-side gasket 109b in the position described above, tolerances occur during the fabrication and assembly of members, and thus it is very difficult to bring the anode-side gasket 109a and the cathode-side gasket 109b into sufficiently close contact with the end faces of the anode 104a and the cathode 104b, respectively. Accordingly, as shown in FIG. 22, when disposing the anode-side gasket 109a and the cathode-side gasket 109b in the position described above, gaps (namely, an anode-side gap 110a and a cathode-side gap 110b) are likely to occur between the anode-side gasket 109a and the anode 104a and between the cathode-side gasket 109b and the cathode 104b. 
If such anode-side gap 110a and cathode-side gap 110b occur, the reaction gases can leak and flow into the anode-side gap 110a and the cathode-side gap 110b. Alternatively, part of the reaction gases flows through the anode-side gap 110a and the cathode-side gap 110b to the outside of the MEA 105, instead of flowing through the anode 104a and the cathode 104b, making it very difficult to maintain power generation performance.
In order to solve the problems, for example, Patent Document 1 proposes a technique in which additional sealants other than the anode-side gasket 109a and the cathode-side gasket 109b are disposed in the anode-side gap 110a and the cathode-side gap 110b so as to fill the gaps.
[Patent Document 1] JP-A-2004-119121